drm/i915/gvt: Return -EIO if host enable_execlists not enabled when loading GVT-g

GVT-g relies on the enable_execlists parameter in i915. If this option
is not enabled for GVT-g, should return -EIO to make i915 driver loading
failed.

v2:
- Use DMR_ERROR as it is a fatal message. (Chris)

Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>